Early Engraftment and Immune Kinetics Following Allogeneic Transplant Using a Novel Reduced-Toxicity Transplant Strategy in Children/Adolescents with High-Risk Transfusion-Dependent Thalassemia: Early Results of the ThalFAbS Trial

Allogeneic hematopoietic stem cell transplantation is challenging for patients with transfusion-dependent thalassemia who have experienced iron overload and received chronic transfusion support. A transplantation strategy including a reduced-intensity preparative regimen and tailored immunosuppression to support donor engraftment and prevent graft-versus-host disease (GVHD) was developed for this population. The combination of a pretransplantation immunosuppression phase with reduced dosing of fludarabine/prednisone, a treosulfan-based preparative regimen with reduced cyclophosphamide dosing, and introduction of a calcineurin/methotrexate-free GVHD prophylaxis/engraftment supporting regimen with abatacept/sirolimus/antithymocyte globulin was tested. In the ThalFAbS trial, a prospective pilot trial (ClinicalTrials.gov NCT05426252) of a transplantation strategy designed for higher-risk patients with transfusion-dependent thalassemia, 12 pediatric patients (4 with alpha thalassemia, 8 with beta thalassemia) were treated with this strategy. Descriptive statistics were used to characterize transplantation outcomes and immune recovery. With a median follow-up of 12 months (range, 4 to 26 months) post-transplantation, all 12 patients had prompt and durable trilineage donor engraftment with low transplantation-related morbidity and acute GVHD and are alive without transfusion support at the time of this report. GVHD was limited to 1 patient with skin-only grade II acute GVHD and 3 patients with limited oral chronic GVHD. Early hematologic and immunologic recovery was achieved, with low rates of transfusion support and infection. Neutrophil recovery occurred at a median of 18 days (range, 15 to 24 days), and platelet recovery occurred at a median of 18 days (range, 12 to 36 days). No patients experienced veno-occlusive disease, transplantation-associated thrombotic microangiopathy, or sepsis. This platform was sufficient to support haploidentical donor transplantation in 2 patients. The ThalFAbS approach is tailored to meet the unique needs of transfusion-dependent thalassemia patients. Delivery of this novel regimen is feasible, and it shows excellent early engraftment and transplantation outcomes. Further follow-up of this cohort and expansion of patient numbers is needed before the findings can be generalized, but early experience is promising.